The invention relates to an optical information medium for rewritable recording by means of a laser-light beam, said medium comprising a substrate carrying a stack of layers, which stack comprises a first, a second and a third dielectric layer, a recording layer of a phase-change material arranged between two dielectric layers, the phase-change material being able to record amorphous marks when in the crystalline state, a light-absorptive layer, and a metal mirror layer.
The invention also relates to the use of such an optical recording medium for land-groove recording.
Optical information or data storage based on the phase-change principle is attractive, because it combines the possibilities of direct overwrite (DOW) and high storage density with easy compatibility with read-only systems. Phase-change optical recording involves the formation of submicrometer-sized amorphous recording marks in a thin crystalline film using a focused laser-light beam. During recording information, the medium is moved with respect to the focused laser-light beam which is modulated in accordance with the information to be recorded. Due to this, quenching takes place in the phase-change recording layer and causes the formation of amorphous information bits in the exposed areas of the recording layer which remains crystalline in the unexposed areas. Erasure of written amorphous marks is realized by recrystallizing through heating with the same laser. The amorphous marks represent the data bits, which can be reproduced via the substrate by a low-power focused laser-light beam. Reflection differences of the amorphous marks with respect to the crystalline recording layer bring about a modulated laser-light beam which is subsequently converted by a detector into a modulated photocurrent in accordance with the coded, recorded digital information.
One of the goals in phase-change optical recording is to obtain a high storage capacity for applications like DVD-RAM, DVD-Rewritable and DVR (Digital Video Recorder), so that such media are suitable for high density recording, e.g. a storage capacity of over 3 Gbyte at a disc diameter of 120 mm. For this purpose, possible options are reducing the laser wavelength .lambda. and/or increasing the numerical aperture (NA), because the laser spot size is proportional to (.lambda./NA).sup.2. An alternative option is the application of multiple recording layers. Another possibility would be to record information tracks both in the grooves and on the lands between the tracking guide grooves of the medium (land-groove recording). These grooves are present to guide the laser-light beam along a track.
The storage density of a phase-change optical information medium is determined by both the radial density and the tangential density of the marks. The radial density is determined by the track pitch, i.e. the distance between adjacent track centerlines of the grooves in a radial direction. The track pitch is limited by thermal crosstalk. This means that the quality of the data recorded in a track will be influenced by recording at the adjacent track. The shape of the recorded marks may then be distorted, which results in a large jitter. The tangential density is determined by the channel bit length, which is limited by the fact that the light absorption of the amorphous state (A.sub.a) is higher than that of the crystalline state (A.sub.c) when a standard IPIM stack is used. In this stack, I represents a dielectric layer, P represents a phase-change recording layer, and M represents a reflective or mirror layer. Therefore, an amorphous portion is heated to a higher temperature than a crystalline portion when the recording film is irradiated with laser light. As a result, the recording marks overwritten in a crystalline area are smaller than those in an amorphous area. Such a phenomenon causes an increase of jitter, which is proportional to the inverse of the channel bit length. To overcome the problem, the difference between A.sub.c and A.sub.a should be minimized, or better A.sub.c .gtoreq.A.sub.a.
An optical information medium of the type mentioned in the opening paragraph is known from U.S. Pat. No. 5,652,036. The known medium of the phase-change type has a substrate carrying a stack of layers comprising three dielectric layers, a phase-change recording layer, a light-absorbing layer, and a reflective layer. Many permutations of possible stacks are disclosed, e.g. an IAIPIM stack, in which I, P, and M have the above mentioned meaning, and A represents the light-absorbing layer. The light-absorbing layer is formed of a mixture comprising a dielectric material and a metal or semiconductor material. The result of the addition of the light-absorbing layer A is that the difference in light-absorption between the amorphous state (A.sub.a) and the crystalline state (A.sub.c) of the recording layer is minimized, thus reducing the recording mark distortion. A disadvantage of the known recording medium is that it is not suitable for high density land-groove recording. This is caused by the fact that the optical phase difference (.PSI..sub.c -.PSI..sub.a) between the crystalline state and the amorphous state in the known stacks is not close to zero, the necessity of which will be explained later.